1. Field of the Invention
This invention relates to teletext decoders for receiving, storing and processing teletext information which is transmitted as digitally coded data and comprises a plurality of different pages each of which is identified by a respective page number. Transmissions of teletext information are in television signals in television lines where no picture signals representing normal television picture information are present. These television lines are referred to as data-lines.
2. Related Art
The document "Broadcast Teletext Specification", September 1976, published jointly by the British Broadcasting Corporation, Independent Broadcasting Authority and British Radio Equipment Manufacturers' Association, discloses a specification for transmitting teletext information in 625-line television systems.
In the above-identified document "Broadcast Teletext Specification", a quantity of teletext information to be considered as an entity is termed a page and will be so termed herein. All of the pages which are available are normally transmitted in a recurrent cycle, with or without up-dating page information, as appropriate. At a teletext decoder any page can be selected, and the digitally coded data representing the page information is then acquired by the teletext decoder from the cyclic transmission and is stored in a page memory of the teletext decoder for as long as the page is required. A teletext decoder may have a multi-page memory having a plurality of memory portions in which individual pages can be stored. These memory portions may be used on a priority basis, that is, if two (or more) memory portions are allotted to store the same selected page, then priority logic in the decoder allows only one portion to receive the page in preference to the other(s).
The pages are organised into different magazines (or groups) and each page consists of up to 24 data rows. The first data row (Row 0) of each page is termed a page-header and contains inter alia the page number. The transmission of each page begins with, and includes, its page-header and ends with, and excludes, the next page-header which is transmitted in respect of a page in the same magazine. Thus, it is assumed that all of the data rows containing the relevant magazine number which are transmitted between two such successively transmitted different page-headers belong to the page having the first page-header.
Proposals for enhancing the teletext specification given in the "Broadcast Teletext Specification" document are given in the document "World System Teletext Technical Specification", March 1985, compiled by the Department of Trade and Industry. One of these enhancement proposals concerns the provision of a conditional access teletext service in which teletext message information in data pages is scrambled prior to transmission, and can only be received as useful information by a teletext decoder having an appropriate descrambling key. Such a descrambling key is itself transmitted as encrypted teletext information in the data page concerned, whilst other keys which are provided to regulate the conditional access to transmitted teletext message information are transmitted in encrypted control pages. Decryption therefore has also to be performed within the teletext decoder.
The reception and processing of the scrambled data pages and the encrypted control pages necessitates the use of a type of teletext decoder which includes processor means for carrying out the descrambling and decryption. The actual reception of the data pages and control pages can be carried out by dedicated hardware circuits of the teletext decoder, albeit under the control of the processor means.
A problem that has been encountered in the realisation of a teletext decoder of this type is that the processor means needs to know when acquisition of a selected teletext page (control or data) has been completed before descrambling or decryption, as the case may be, of the teletext page can commence. This problem occurs because although the aforementioned page-header feature provides a specific page-found indication from which a `page-found` signal can be produced directly to signify the start of a selected page, this feature does not provide a specific end-of-page indication at the end of the transmitted page from which an end-of-page signal can be produced directly at the end of a transmitted page.